Takaaki Morimoto

Defects in Yttria-Stabilized Zirconia Induced by Irradiation of Ultraviolet Photons

When single-crystal yttria-stabilized zirconia samples are exposed to ultraviolet photons with an energy higher than 4.0 eV, three paramagnetic centers are observed at g=2.006, 1.97–1.95, and 1.91–1.86. With irradiation by ultraviolet photons, an absorption band and a photoluminescence (PL) band are also induced, showing respective peak energies at around 3.3 and 2.8 eV. The onset of the PL-excitation spectrum occurs around 4.0 eV. The induced absorption band and paramagnetic centers disappear with thermal annealing. By doing a numerical analysis on the experimentally obtained angular dependence of the signal at g=1.91–1.86, it is determined that signals at g=2.006 and 1.91–1.86 are due to the F+ center and the T center, respectively, and that the signal at g=1.97–1.95 has some origin other than the two ESR centers. Since two ESR signals at g=2.006 and 1.97–1.95, the 3.3-eV absorption, and the 2.8-eV PL have the same onset energy at around 4.0 eV, the reactions that induce them are triggered by electrons excited into the conduction band tail.

Cause of the appearance of oxygen vacancies in yttria-stabilized zirconia and its relation to 2.8 eV photoluminescence

When we implanted P+ or B+ ions into yttria-stabilized zirconia (YSZ), its crystallinity was degraded. Concurrently, the photoluminescence at around 2.8 eV originating from two types of oxygen vacancies with one or two captured electrons became weak, indicating a decrease in the number of oxygen vacancies. Oxygen vacancies appear in YSZ as a result of the replacement of Zr4+ by Y3+ in ZrO2. Therefore, it seems that the separation of YSZ into ZrO2 and Y2O3 induced by the ion implantation is responsible for the decrease in the number of oxygen vacancies. Moreover, the intensity of the 2.8 eV photoluminescence returns to the value before the ion implantation if the sample is annealed thermally after the implantation at temperatures higher than the crystallization temperature of YSZ. The reaction opposite to the above seems to be induced by the thermal annealing.

Roles of Point Defects in Thermally Enhanced Generation and Transfer of Electrons and Holes in LaAlO3

Thermal annealing was given to single crystal LaAlO3 and its effects were examined by measuring electron spin resonance (ESR) and optical absorption. When LaAlO3 was annealed at temperatures above 500 °C in an oxidizing atmosphere, the intensities of ESR signals due to transition metal, likely ascribable to Fe3+, decreased. Concurrently with this, two optical absorption bands at 2.7 and 3.5 eV, attributable to a combination of a hole and a La3+ (or Al3+) vacancy, increased. These results indicate that thermal electron–hole generation is induced by oxidizing annealing and that the generated electrons and holes are then captured by Fe3+ ions and La3+ or Al3+ vacancies, respectively. It is also assumed that captured electrons and holes are released and recombine with each other by reducing annealing.

Dielectric absorption behavior of YAlO3 at terahertz frequencies

Using THz time-domain spectroscopy, optical or dielectric absorption spectra were obtained for YAlO3(100) single crystals. A sharp absorption peak appears at around 3.4 THz only when the THz electric field is parallel to the samples [011] axis. This peak should be due to a normal mode of vibration, to which Y largely contributes.

Crystalline structures of YAlO 3 single crystal at high temperatures

In order to clarify the influence of high temperature annealing on a next-generation gate insulating material YAlO₃, single crystal YAlO₃ samples were annealed at various temperatures from 900 to 1300°C in air for about 12 hours. The crystalline structure was examined by X-ray diffractometry, while the surface profile was examined by atomic force microscopy and a surface profilometer. Furthermore, infrared absorption spectroscopy and optical microscopy were used. As a result, the following serial changes in structure were estimated. The perovskite structure of crystalline YAlO₃ starts to collapse and Y₃Al₅O₁₂ (YAG) is formed by the annealing at 1160 °C. Then, it segregates to Al₂O₃ andYAlO₃ after the annealing at 1300 °C.

Structural change induced in LaAlO 3 by ion implantation

Ion implantation is used for various purposes in manufacturing semiconductor devices such as MOS-FETs. In the present study, effects of implantation of P+ or B+ ions on the structural change of single crystal LaAlO3 were examined. The optical absorption edge located at about 5.6 eV, which corresponds to the band gap energy of LaAlO3, is scarcely affected by the ion implantation. The X-ray diffraction peak intensity at 2θ= 23.5° is decreased by the ion implantation. The intensities of three sharp photoluminescence (PL) peaks detected at 1.62, 1.65, and 1.69 eV, which appear only when the samples are crystalline, become smaller by ion implantation. However, the intensity of a broad PL peak at around 2.8 eV due to the oxygen vacancy, which is also detectable in amorphous samples, scarcely changes after the ion implantation. These results indicate that the ion implantation degrades the crystalline LaAlO3. However, as compared with YAlO3 with a similar perovskite structure, LaAlO3 is more resistant to ion implantation.